The converter is fine. He said there is 12V power when on shore power. The simple answer is the battery is not connected to the system. The jack and slides work because they have more direct connections. There’s still another bad fuse, breaker or connection between the battery and 12V load center. Get a volt meter or test light and start poking around. The battery cutoff switch could have failed. Try jumping across it if you can’t check for voltage at it.
The 2 fuses by the battery are probably for the jack and slide that work. The ones in the converter are for when it’s charging. As enblethen suggested, there probably another fuse or breaker.